Polyphenylene sulfide fibers (hereinafter may be referred to as PPS fibers) are excellent in heat resistance and chemical resistance and their application as high-function fibers has increasingly expanded. Specific application thereof include filters used for collecting dusts in high temperature gas, dryer fabric (canvas) in a drying step for industrial products, and roll wiping materials for office copy machines. The area of their application will be further expanded.
PPS fiber paper, inter alia, has been adopted as roll wiping materials for office copy machines. Such copy machines adopting PPS fiber paper as roll wiping materials are increasing because PPS fiber paper is excellent in lightness and flexibility, which are typical characteristics of paper, and in wiping performance. It has been reported that the use of a fiber having crimps as a PPS fiber could produce PPS fiber paper which has a high sheet strength even with a low mass per unit area and which was also dense and uniform (see Patent Literature 1). Patent Literature 1 proposes specific application of PPS fiber paper to heat-resistant electrical insulation materials, battery separators, and the like.
In recent years, the energy densities of secondary batteries represented by nickel-hydrogen batteries and lithium ion batteries have dramatically increased. Further, for on-vehicle battery application, such batteries with higher capacities have been rapidly developed.
Electrical insulation materials used in batteries, motors, inverters, and the like have been under heavy demand in terms of their performance. For example, a motor insulation material for insulating a winding from a stator or a rotor is sometimes impregnated with a resin varnish in order to further increase its insulation performance. Thus an insulation material sheet subjected to impregnation with a varnish is required to have an excellent property of allowing impregnation with a varnish solution. Further, a secondary battery having a high energy density is exposed to a high temperature environment, and consequently dew condensation may occur due to the humidity in the air. In order to prevent reduction in secondary battery performance due to dew condensation, an insulation material used for the battery is required to exhibit a stable moisture proof effect that does not allow moisture from dew condensation to penetrate through the insulation material. That is, electrical insulation materials are required to have two properties: an excellent property of allowing impregnation with a varnish solution and a property of preventing moisture penetration. Further, other properties required for electrical insulation materials include a property of dissipating heat in order to allow heat generated from a coil winding to easily dissipate in the atmosphere, thereby suppressing an increase in the temperature of an instrument that is being used.
As described above, motor insulation materials are required to have a property of allowing impregnation with a varnish, a property of preventing moisture penetration, a property of dissipating heat, and the like. Patent Literature 1 discloses that a wet-laid nonwoven fabric made of a PPS fiber (a nonwoven fabric obtainable by a process for papermaking) may comprise a binder and that the wet-laid nonwoven fabric is run through a calender to be heated and pressed. However, the invention described in Patent Literature 1 fails to offer a sufficient solution to the above demands and problems, in particular, a solution to provide a wet-laid nonwoven fabric (a nonwoven fabric obtainable by a process for papermaking) with an excellent property of allowing impregnation with a varnish solution and a property of preventing moisture penetration thereinto.
Another proposed technology suitable for a highly heat-resistant electrical insulation sheet is a heat-resistant nonwoven fabric produced by blending a heat-resistant fiber and an undrawn polyphenylene sulfide fiber (see Patent Literature 2). In Patent Literature 2, the undrawn polyphenylene sulfide fiber in a blending ratio of less than 8% is subjected to thermal fusion bonding. As a result of that, the density of the heat-resistant nonwoven fabric is prevented from becoming too high and the resulting paper is prevented from becoming too smooth and too thin like a sheet, which in turn improves a property of allowing impregnation with a varnish. The invention described in Patent Literature 2 is obtainable by carding a short PPS fiber to form a web and thermally fusion bonding it under a pressure of lower than 100 kg/cm, which pressure is predetermined to prevent the density of the heat-resistant nonwoven fabric from becoming too high. The nonwoven fabric obtainable by the method described in Patent Literature 2 has an excellent property of allowing impregnation with a varnish but fails to provide a complete solution for maintaining a property of allowing impregnation with a varnish while preventing moisture penetration. Further, Patent Literature 2 discloses that the nonwoven fabric is obtainable by a dry method such as a carding method or an air-laid method and that the density of the fabric is preferably not too high. However, in a carding method or an air-laid method, a crimped short fiber having a fiber length of 38 mm or more is usually used for producing a nonwoven fabric, and consequently an obtained web is bulky and the dispersion state of the fiber is inferior to that in a nonwoven fabric obtainable by a process for papermaking. Therefore, unlike a nonwoven fabric obtainable by a process for papermaking of the present invention, the nonwoven fabric disclosed in Patent Literature 2 lacks the uniformity of the formation and thinness and thus fails to provide a solution for achieving both of an excellent property of allowing impregnation with a varnish and a property of controlling moisture penetration as well as a solution for achieving lightness required for an electrical insulation material.
Known another technology is a printing paper obtainable by a wet method (see Patent Literature 3). Patent Literature 3 discloses that application of moisture to paper during calendering treatment following papermaking using pulp increases the degrees of opacity, white glossiness, and smoothness of the paper. Specifically, the calendering treatment is performed on both surfaces of a nonwoven fabric using a hot soft nip calender preferably having 6 or more nips.
Further, a PPS fiber nonwoven fabric obtainable by a hitherto known process for papermaking has lightness and flexibility, which are typical characteristics of paper, and heat resistance; however, it lacks an excellent property of allowing impregnation with a varnish, which property is required for an electrical insulation material, and excellent dimensional stability in a high temperature environment such as moist heat or dry heat.